There are three principal beds of obsidian, of which
the thickest forms the base of the section. The alternating stony layers
appear to me eminently curious, and shall be first described, and
afterwards their passage into the obsidian. They have an extremely
diversified appearance; five principal varieties may be noticed, but these
insensibly blend into each other by endless gradations.

FIRST.

A pale grey, irregularly and coarsely laminated (This term is open to some
misinterpretation, as it may be applied both to rocks divided into laminae
of exactly the same composition, and to layers firmly attached to each
other, with no fissile tendency, but composed of different minerals, or of
different shades of colour. The term "laminated," in this chapter, is
applied in these latter senses; where a homogeneous rock splits, as in the
former sense, in a given direction, like clay-slate, I have used the term
"fissile."), harsh-feeling rock, resembling clay-slate which has been in
contact with a trap-dike, and with a fracture of about the same degree of
crystalline structure. This rock, as well as the following varieties,
easily fuses into a pale glass. The greater part is honeycombed with
irregular, angular, cavities, so that the whole has a curious appearance,
and some fragments resemble in a remarkable manner silicified logs of
decayed wood. This variety, especially where more compact, is often marked
with thin whitish streaks, which are either straight or wrap round, one
behind the other, the elongated carious hollows.

SECONDLY.

A bluish grey or pale brown, compact, heavy, homogeneous stone, with an
angular, uneven, earthy fracture; viewed, however, under a lens of high
power, the fracture is seen to be distinctly crystalline, and even separate
minerals can be distinguished.

THIRDLY.

A stone of the same kind with the last, but streaked with numerous,
parallel, slightly tortuous, white lines of the thickness of hairs. These
white lines are more crystalline than the parts between them; and the stone
splits along them: they frequently expand into exceedingly thin cavities,
which are often only just perceptible with a lens. The matter forming the
white lines becomes better crystallised in these cavities, and Professor
Miller was fortunate enough, after several trials, to ascertain that the
white crystals, which are the largest, were of quartz (Professor Miller
informs me that the crystals which he measured had the faces P, z, m of the
figure (147) given by Haidinger in his Translation of Mohs; and he adds,
that it is remarkable, that none of them had the slightest trace of faces r
of the regular six-sided prism.), and that the minute green transparent
needles were augite, or, as they would more generally be called, diopside:
besides these crystals, there are some minute, dark specks without a trace
of crystalline, and some fine, white, granular, crystalline matter which is
probably feldspar. Minute fragments of this rock are easily fusible.

FOURTHLY.

A compact crystalline rock, banded in straight lines with innumerable
layers of white and grey shades of colour, varying in width from the
thirtieth to the two-hundredth of an inch; these layers seem to be composed
chiefly of feldspar, and they contain numerous perfect crystals of glassy
feldspar, which are placed lengthways; they are also thickly studded with
microscopically minute, amorphous, black specks, which are placed in rows,
either standing separately, or more frequently united, two or three or
several together, into black lines, thinner than a hair. When a small
fragment is heated in the blowpipe, the black specks are easily fused into
black brilliant beads, which become magnetic,--characters that apply to no
common mineral except hornblende or augite. With the black specks there are
mingled some others of a red colour, which are magnetic before being
heated, and no doubt are oxide of iron. Round two little cavities, in a
specimen of this variety, I found the black specks aggregated into minute
crystals, appearing like those of augite or hornblende, but too dull and
small to be measured by the goniometer; in the specimen, also, I could
distinguish amidst the crystalline feldspar, grains, which had the aspect
of quartz.